The present invention relates to a continuously variable transmission having a hydraulic continuously variable transmission (hereinafter referred to as an xe2x80x9cHSTxe2x80x9d). Especially, it relates to a hydraulic and mechanical composite transmission (hereinafter referred to as an xe2x80x9cHMTxe2x80x9d) as a combination of an HST and a mechanical speed change mechanism having a differential part as a planetary gearing, wherein the mechanical speed change mechanism is constructed such that the output rotation of the HST and the rotation of its input part receiving engine power are transmitted into the differential part, and then, the differential rotation of the differential part is transmitted to a speed change output shaft part thereof. Further especially, it relates to that have the mechanical speed change mechanism provided with setting means which can electrically set a rotational speed ratio of the speed change output shaft part to the speed change input shaft part. This ratio is referred to as a speed ratio.
Conventionally, there is a well-known HMT as a combination of an HST and a mechanical speed change mechanism, wherein the mechanical speed change mechanism is constructed such that the rotational force of an input part thereof for receiving engine power (a speed change input part) and the output rotational force of the HST are transmitted to a differential part thereof having a group of planetary gears and the differential rotation of the group of planetary gears in the differential part is transmitted to a speed change output part thereof.
In the conventional HMT, the input part of the differential part for receiving engine power is constructed separately from a pump shaft of the HST. Referring to the above art, an input shaft for the whole HMT serves as a rotational axis of the differential part. However, power from the differential part is transmitted to a pump shaft of the HST through a gear train, thereby detracting efficiency of transmission. Furthermore, the pump part of the HST cannot be disposed coaxially to the differential part, thereby inhibiting minimization of the HMT. Furthermore, as well-known from U.S. Pat. No. 4,259,881, the differential part is radially expanded because it includes a ring gear having an inner peripheral gear, thereby also inhibiting minimization of the HMT.
In the conventional HMT, as well-known, power can be selectively transmitted to the output part thereof from either a motor shaft of the HST (hereinafter, such a transmission mode is referred to as an xe2x80x9cHST modexe2x80x9d) or the differential part (hereinafter, such a transmission mode is referred to as an xe2x80x9cHMT modexe2x80x9d). For transmission of engine power to the output part with the least loss, it is desirable that a gear is interposed between the input part and the output part in the HMT so as to transmit power without the HST and the differential part. In other words, the power transmission with such a gear train may be selected during a high-speed traveling or so on. However, such a gear train is not provided in the conventional HMTs. Of course, there is no control system for selecting the transmission with such a gear train.
As for control of the HMT, an electromagnetic clutch is conventionally used for changing a transmission mode between the HST mode and the HMT mode. A certain speed ratio is set for determining the timing of this mode change. However, in the conventional clutch during its disengagement, only one clutch side is rotated while the other is stationary. Thus, when the clutch is engaged, the stationary clutch side resists against the rotating clutch side, thereby not only changing the rotating speed suddenly but also stressing the abutting surfaces of the clutch greatly. Therefore, the conventional clutch is made of strong material such as sintered metal, which is expensive and enlarged so as to inhibit its minimization.
As for the change of transmission mode, if a lag in completing the operation of the electromagnetic clutch, that is caused by transmitting an electric output signal to the electromagnetic clutch and a time of actual operation of the electromagnetic clutch, is not considered, speed is suddenly changed because of a difference of output/input speed ratio between pre-operation and post-operation of the clutch. The above-mentioned conventional art does not consider adjustment of the output/input speed ratio, or the timing, for changing the transmission mode.
If a brake is provided on the output part, the braked output part resists against the transmission system from the differential part or the motor shaft of the HST, thereby damaging a clutch between the output part and the transmission system. The above-mentioned conventional art does not consider a relationship between the brake and the clutch.
The adjustment of the output/input speed ratio of the HMT depends upon the adjustment of the amount of oil discharged from the HST. In a composition of two variable displacement hydraulic units shown in U.S. Pat. No. 5,421,790, the actuations of both the hydraulic units are sequential. For example, when one hydraulic unit discharges, the other hydraulic unit is in neutral, i.e., the functions of both the hydraulic units as a hydraulic pump and a hydraulic motor are exchanged with each other in respective ranges of speed ratio. However, if speed ratio setting means is to be greatly shifted so as to greatly change the speed ratio, it takes a long time to change the speed reduction under such hydraulic control so that a vehicle is uncomfortably accelerated or decelerated. If both the hydraulic units were adjusted in their discharge simultaneously, a target speed ratio could be attained soon. However, if the speed ratio is varied across a point for changing the transmission mode, the clutch is greatly stressed under the situation where both the hydraulic units are actuated.
If load is applied, the output part is decelerated by the load so that the actual speed ratio becomes different from the set speed ratio. In this case, usually, an engine rotation is adjusted by a governor. The conventional HMT does not consider the speed ratio to be adjusted for amendment of output rotation. If the speed ratio is to be adjusted, a relation of actuation between both the hydraulic units constituting the HST must be considered. Furthermore, if the actual speed ratio is changed according to such variance of load while a speed ratio is set in the vicinity of the change point of transmission mode, the transmission mode is frequently alternated, thereby damaging the clutch and making the travel of a vehicle unstable.
Regarding a continuously variable transmission including an HST (whether it may be an HMT or constituted only by an HST), in the case where the speed ratio is controlled electrically, e.g., correspondingly to a voltage issued from a position sensor which detects a position of a lever for setting a speed ratio, it is assumed that the variance ratio of the speed ratio is constant in its whole range to be set. On this assumption, if the variance ratio is set in correspondence to a low speed range, the lever must be shifted to a considerably large degree for establishing a high speed. If the variance ratio is set in correspondence to a high speed range, the speed ratio is greatly varied in a low speed range while the lever is shifted to a small degree so that the low speed travel of a vehicle during a work or so on becomes unstable.
Furthermore, conventionally, such an electric speed ratio control system does not have control means for adjusting acceleration and deceleration in corresponding to the speed of shifting.
Conventionally, for setting the speed ratio to zero, the discharge of the variable displacement hydraulic unit of the HST is set to zero. However, hydraulic control for establishing a neutral condition is difficult. Even if oil extremely slightly escapes, a motor shaft is, so far from certainly stationary, moved. Especially, such a problem arises when a vehicle is stopped on a slope. For solving this problem, for example, biasing means for neutral returning is provided on capacity setting means of the HST. However, this structure is complicated and increases manufacturing costs. If the capacity setting means is electrically controlled, by contriving an electric control signal in neutral, the vehicle may be enabled to stop securely without complicating the capacity setting means.
Conventionally, there is a well-known governor for controlling engine rotation wherein a load regulation mode for varying the engine rotation in spite of setting an accelerator can be established according to detection of load on an engine so as to increase the resistance against the load during excessive loading, and to reduce engine noise during light loading. During the load regulation mode, it happens that the actual traveling speed is different from the speed set by the accelerator. For compensating the difference, it may be thought that the speed reduction of the HMT is adjusted. Conversely speaking, such a compensation enables the engine rotation to be regulated in the load regulation mode. However, the variance of engine rotation causes a variance of rotational speed of a PTO shaft, and a great variance of traveling speed in a middle-and-high speed range. Therefore, a situation must be limited.
Conventionally, the continuously variable transmission comprising an HST or so on, which can electrically control a speed ratio, is not constructed such that a left-and-right turning radius is varied according to the change of the set speed ratio in correspondence to the turning angle of a steering wheel so as to enable a vehicle to turn on a small circle.
An object of the present invention is to provide a continuously variable transmission having an HST (e.g., an HMT), which is improved so as to solve the above-mentioned problems.
First, to provide a compact HMT having high efficiency in its transmission of power from an engine, the present invention is so constructed as follows:
An HST and a mechanical speed change mechanism which includes a differential part constituted by a group of planetary gears are interposed between a speed change input part for receiving power from a prime mover and a speed change output part. The rotation of the speed change input part is transmitted to both a hydraulic pump of the HST and a first differential input part of the differential part. The differential part further includes a second differential input part for receiving the rotational force of a hydraulic motor of the HST. The rotation of the group of planetary gears generated by the difference of rotational speed between the first and second differential input parts can be transmitted to the speed change output part. In such a structure of the HMT, a pump shaft of the HST is formed on its one end side with the speed change input part, and on the other end side with the first differential input part coaxial with the speed change input part. The first differential input part may be structured by extending the pump shaft.
Furthermore, the first differential input part is provided with an outer-peripherally toothed first sun gear engaging with the group of planetary gears. The second differential input part is provided with an outer-peripherally toothed second sun gear freely rotatably disposed on a shaft of the first sun gear so as to engage with the group of planetary gears. Consequently, the differential part having no inner-peripherally toothed ring gear can be further minimized.
Still furthermore, a shaft serving as the first differential input part is extended so as to join a PTO shaft so that the parts between the speed change input part and the PTO shaft can be coaxially disposed, thereby enabling a compact transmission to be provided.
The mechanical speed change mechanism of the HMT is provided with first, second and third drive trains. The first drive train is interposed between the motor shaft and the second differential input part. The second drive train transmits the differential rotation of the group of planetary gears to the speed change output part. The third drive train transmits the rotation of the motor shaft to the speed change output part without passing the differential part. As for a transmission mode of the HMT, the HMT can be selectively placed into either a first transmission mode or a second transmission mode. In the first transmission mode, power is transmitted from the motor shaft to the speed change output part through the third drive train while the first drive train is isolated. In the second transmission mode, power is transmitted from the motor shaft to the speed change output part through the first drive train, the differential part and the second drive train while the third drive train is isolated. Referring to a ratio of rotational speed of the speed change output part to that of the speed change input part as a speed ratio, the hydraulic pump or the hydraulic motor is adjusted in its discharge amount so as to change the speed ratio.
In the above-mentioned structure of the HMT according to the present invention, since a timing for altering the transmission mode during an operation for changing the speed ratio is a time when the rotational speed of the group of planetary gears which are freely rotated by rotation of the speed change output part during the first transmission mode substantially coincides with the rotational speed of the same during the second transmission mode, the vehicle is prevented from being suddenly varied in its traveling speed when the transmission mode is exchanged.
Furthermore, in the case where such a timing for altering the transmission mode is set, a first clutch is interposed on the first drive train, and a second clutch on the second drive train. The first transmission mode is established by disengaging the first clutch and engaging the second clutch, and the second transmission mode by engaging the first clutch and disengaging the second clutch. Since the transmission mode exchanging timing is set at the time when the rotational speed of the group of planetary gears in the first transmission mode coincides with that in the second transmission mode as mentioned above, the clutches are prevented from being stressed during their engaging. Consequently, the strength of the clutches can be reduced so as to reduce their manufacturing costs and minimize them.
In this structure of the HMT, the transmission mode change timing corresponds to a certain speed ratio in a range of speed ratio to be set for forward traveling is set. When the speed ratio set for forward traveling is less than the certain speed ratio or when any speed ratio in its whole range to be set for rearward traveling is set, the HMT is placed into the first transmission mode. When the speed ratio set for forward traveling is more than the certain speed ratio, the HMT is placed into the second transmission mode. Then, the HMT is controlled in the speed ratio. Consequently, if the vehicle placed in the first transmission mode travels forward at low speed or rearward, high torque fitting a work by the vehicle can be obtained. If the vehicle placed in the second transmission mode travels forward at middle or high speed, a speed ratio can be selected (a traveling speed is changed) under efficient transmission with reduced loss in correspondence to the normal traveling. Furthermore, since the whole speed range for rearward traveling is established according to the first transmission mode, the traveling direction of the vehicle in the second transmission mode is only forward, thereby requiring no reversing drive train to be interposed between the differential part and the speed change output part in the HMT.
Furthermore, in order to provide a transmission system further improved in its transmission efficiency for establishing high speed, a four drive train is provided for transmitting power from the speed change input part to the speed change output part through neither the HST nor the differential part. A third transmission mode for transmitting power from the speed change input part to the speed change output part through the fourth drive train while the first and second drive trains are isolated from transmission is provided to the HMT. If the vehicle is placed in the first or second transmission mode, the four drive train is isolated from transmission.
In this structure, in order to prevent sudden speed variance during the exchange of transmission mode between the third transmission mode and the second transmission mode, when the maximum speed ratio for high speed forward traveling is set, the rotational speed of the speed change output part during the second transmission mode substantially coincides with the rotational speed of the speed change output part during the third transmission mode, and the second transmission mode and the third transmission mode are exchanged with each other.
A third clutch is interposed on the fourth drive train in addition to the first and second clutches while the timing for exchanging the transmission mode between the second and third transmission modes. Consequently, the stress applied on the third clutch during its operation can be reduced so as to reduce its manufacturing cost and minimize it.
Incidentally, the HMT in the third transmission mode is short of resistance against load. Therefore, a means for detecting the rotational speed of the prime mover is provided. The HMT is placed into the third transmission mode only when the rotational speed of the prime mover detected at the period for exchange between the second and third transmission modes is in or adjacent to a range between its rotational speed corresponding to the maximum torque of the prime mover and its rotational speed corresponding to the maximum output of the prime mover.
For amendment of the transmission mode change timing, each of the clutches is electrically controlled. A lag for electric operation and a lag for mechanical operation are computed. Referring to the sum of both the lags as an amendment time, the transmission mode change timing is advanced at the amendment time so that just when the exchange of clutches is completed, a speed ratio fitting the transmission mode change timing is reached, thereby preventing sudden variance of speed during the exchange of clutches.
During an operation of changing the speed ratio, a speed ratio is momently detected. It is computed how much the detected speed ratio becomes when the amendment time computed as mentioned above has passed. When this computed speed ratio is decided to become the speed ratio corresponding to the transmission mode change timing, the transmission mode is exchanged at this time.
When a brake for braking the speed change output part is operated, all the clutches on the first and second drive trains (and the four drive train, if it is provided) are disengaged so as to prevent the driving force from being applied together with the braking force onto the speed change output part, thereby smoothing the braking and protecting the clutches and other parts.
When the speed change output part is released from the braking condition with the braking means, the ratio of discharge amount of the hydraulic pump to that of the hydraulic motor in the HST is regulated to make the actual speed ratio agree with the set speed ratio, and then, one of all the clutches is engaged so as to revive one of the first and second transmission modes (and the third transmission mode). Due to this order, the clutches and other parts can be protected during the release of brake, and the traveling speed can be revived smoothly.
For setting a speed ratio, a speed ratio setting means manipulated by an operator is provided. A position of the speed ratio setting means is detected and a target speed ratio is set in correspondence to the detected position. Both the hydraulic pump and the hydraulic motor in the HST are of a variable displacement type. The range for setting a speed ratio comprises a pump control zone for varying only discharge amount of the hydraulic pump and a motor control zone for varying only discharge amount of the hydraulic motor. The pump control zone and the motor control zone are continuous to each other. If the boundary speed ratio between the pump control zone and the pump control zone is between the target speed ratio and the actual speed ratio, both the hydraulic pump and the hydraulic motor are simultaneously regulated in their discharge amount so that the actual speed ratio quickly reaches the target speed ratio in correspondence to the large shift, or the large difference in speed ratio between actual and target, thereby establishing desirable acceleration or deceleration.
Also, in such a manner that the increase or decrease of speed is accelerated when the degree of shift is as large as over the different speed ratio setting ranges, if the boundary speed ratio between the target speed ratio and the actual speed ratio, and the speed ratio corresponding to the transmission mode change timing are between the pump control zone and the motor control zone, the speed ratio corresponding to the transmission mode change timing is set as a provisional target speed ratio, and then, the discharge amounts of both the hydraulic pump and the hydraulic motor are simultaneously varied. Due to this order of operation, the problem that the clutches are actuated during the simultaneous variance in discharge amounts of both the hydraulic pump and motor can be prevented, thereby making it possible to protect the clutches and the like and to prevent sudden variance of speed ratio.
Furthermore, when the speed ratio is varied across the speed ratio corresponding to the transmission mode change timing, the transmission mode is not altered unless the voltage from the means for detecting the position of the speed ratio setting means is varied really with manipulation of the speed ratio setting means by an operator. Thus, even if the actual speed ratio becomes different from the set speed ratio so as to enter one speed ratio setting range other than that to which the set speed range belongs, the transmission mode is not exchanged so as to prevent the problem that the transmission mode is automatically altered frequently, thereby protecting the clutches and the like and stabilizing the vehicle in travel.
Next, regarding a continuously variable transmission like an HMT, that is provided with an HST having an electrically controlled speed ratio regulating means for varying at least a movable swash plate of its hydraulic pump (and that of its hydraulic motor, if required,), the variance ratio of the target speed ratio to the shift degree of the speed ratio setting means is not constant but varied in correspondence to each speed ratio setting range. For example, the variance ratio is held down in a low speed range for work by the vehicle so that the traveling speed is delicately adjusted by slight shift, thereby enabling fine work. On the other hand, the variance ratio is increased in a high speed range for a normal travel of the vehicle so that the vehicle can be accelerated and decelerated fittingly to its normal travel only by slight shift.
Furthermore, the shift speed of the speed ratio setting means is computed. Then, an amendment value is computed from the shift speed and the present target speed ratio corresponding to the real position of the speed ratio setting means, and added to or subtracted from the present target speed ratio so as to serve as a provisional target speed ratio. Accordingly, for example, if the speed ratio setting means is shifted fast for acceleration, the provisional target speed ratio is set considerably higher than the present target speed ratio set by the speed ratio setting means so that the speed ratio is increased to this provisional target speed ratio, thereby further accelerating the increase of traveling speed. In brief, if the traveling speed is desired to increase or decrease quickly, the speed ratio setting means is shifted fast while the shift degree thereof is slight, thereby enabling desirable acceleration or deceleration.
Suppose that the speed ratio set by the speed ratio setting means is 0. If the actual speed ratio is in an extremely low speed range for forward travel or for rearward travel, the target speed ratio is amended so as to reverse the rotational direction of a motor shaft of the HST. Thus, while the vehicle stands still, the HST repeats oil discharge for the forward travel and for the rearward travel alternately. Even if the vehicle is brought into stationary while being oriented downwardly on a slope, oil is circulated in the direction for the rearward traveling in the HST, thereby preventing its output rotation directed for forward traveling. On the contrary, even if being oriented upwardly on the slope, the output rotation of the HST directed for rearward traveling is prevented by the oil circulation for the forward traveling in the HST. Whichever is the case, oil circulation directed to prevent the descend of the vehicle on the slope is intermittently established in the HST, thereby surely holding the vehicle in stationary. Such a neutral condition of the HST can be obtained by electrically controlling the oil discharge from the variable displacement hydraulic unit in the HST instead of a conventional neutral-returning means like a spring, thereby reducing the manufacturing cost of the HST.
In this case, at least one of the hydraulic pump and the hydraulic motor is of a variable displacement type that is volumetrically regulated by tilting its swash plate. The amendment value of the target speed ratio is the minimum variance of angle of the movable swash plate so as to extremely diminish the oil leak while the speed ratio is 0. Consequently, the vehicle is prevented from mincingly moving. Even while such amendment value is set, it is sufficiently effective for preventing reckless driving of the vehicle on a slope or the like so as to surely hold the vehicle in stationary.
Suppose a vehicle having the continuously variable transmission is provided with a prime mover rotation control device (a governor) which can be placed in a load control mode for controlling the rotational speed of a prime mover in correspondence to the magnitude of load applied on the prime mover. When the prime mover rotation control device placed in the load control mode changes the rotational speed of the prime mover in correspondence to load on the prime mover, the speed ratio is controlled by the continuously variable transmission so as to adjust the rotational speed of the speed change output part to a rotational speed fitting the set traveling speed so that the traveling speed can be maintained. Consequently, the vehicle can travel while at work at desirable constant speed and in the driving condition corresponding to load on the prime mover.
Furthermore, if this vehicle is provided with a PTO portion which is rotated by power of the prime mover at least in upstream from the speed change input part, and then the prime mover rotation control device is placed in the load control mode, the prime mover is not controlled in its rotation according to the selected load control mode unless power is transmitted to the PTO portion. Consequently, the load control mode, in which there are generated a variance of rotational speed of the PTO portion causing rough work and an uncomfortable difference of the rotational speed of the engine from that set by an accelerator, is limited.
Furthermore, the hydraulic motor in the HST is of a variable displacement type, as well as the hydraulic pump. If the actual speed ratio is different from the speed ratio set by the speed ratio setting means while the speed ratio setting means is out of operation for changing a speed ratio, first, the hydraulic pump is regulated in its discharge amount. Until the discharge amount of the hydraulic pump reaches its maximum, the hydraulic motor is not regulated in its discharge amount for making the actual speed ratio agree with the set speed ratio. Therefore, when the traveling speed is varied because of load on the engine or the like, the traveling speed can be held at the set speed because the speed ratio is controlled by the transmission without depending upon the engine control by the governor.
In association with steering, a steering operation means and a rotary means which is varied in its rotational speed in opposite directions correspondingly to the operational direction and degree of the steering operation means are provided. The rotation of the rotary means as well as the rotation of the speed change output part of the continuously variable transmission having an HST such as an HMT is transmitted to an axle differential device so as to differentially drive left and right drive wheels, thereby making the vehicle turn left or right. The speed ratio is decreased according to the increase of operational degree of the steering operation means. When the operational degree of the steering operation means becomes adjacent to the maximum, the rotation of the speed change output part stops. Consequently, by operating the steering operation means at a large degree, the vehicle can turn on a small circle and be gradually brought into spin-turn. Finally, when the steering operation degree reaches the maximum so as to stop the speed change output part, the vehicle can spin-turn on the minimum circle. Due to such a movement during steering, the vehicle can turn on a small butt. In other words, instead of the trouble to operate the speed ratio means for deceleration during the steering, only by extremely fully operating the steering operation means, the vehicle can be naturally decelerated and then spin-turn, thereby facilitating its operation for steering.
Furthermore, this control of speed ratio in correspondence to the operational degree of steering is not performed unless the traveling speed of the vehicle is less than a certain speed. Alternatively, it can be selected whether this control is performed or not. Therefore, the problem is prevented that the vehicle unexpectedly decelerates and spin-turns.